Hot-dip zinc alloy coated steel products

ABSTRACT

Hot-dip zinc alloy coated steel products coated with a Zn-Al-Si-Mg alloy having a composition of between 3.5 and 5.0 wt % aluminum, between 0.02 and 0.5 wt % silicon, between 0.01 and less than 0.05 wt % magnesium, and remainder zinc with unavoidable impurities.

DETAILED EXPLANATION OF THE INVENTION BACKGROUND OF THE INVENTION

This invention relates to hot-dip zinc alloy coated steel productshaving improved resistance to corrosion and good adhesive property ofthe alloy coatings, to methods of their manufacture and to fluxcompositions used in the manufacture.

Zinc alloy coatings have long been applied to steel products andcompared to electroplating the hot-dip zinc alloy coating can be appliedto structurally complicated steel products such as pipes and tubes,section steel, and shaped or contoured parts, such as bolts and nuts,and of course can also be applied to steel sheets and steel wires, etc.,at a low cost. For this reason, the use of hot-dipping has become verypopular. On comparison with electrogalvanized steel products produced byelectroplating, however, hot-dip zinc alloy coated steel productsproduced by the hot dipping process are found to be inferior in theadhesion of the coating to the steel surface, and for this reason, inworkability and further in corrosion resistance.

With the object of improving defects of hot-dip zinc alloy coated steelproducts, and of improving the corrosion resistance of the coatings, inparticular, many propsals have been made on hot-dip zinc alloy coatedsteel products.

In the Japanese Patent Publication (before examination) No. Tokkaisho50-104731 (1975), a Zn-Al-Si alloy for hot-dip coating having acomposition of 5-20 wt% aluminium (Al), less than 5 wt% silicon (Si) andremainder zinc (Zn) is proposed and in the Japanese Patent Publication(before examination) No. Tokkaisho 54-23032 (1979), a Zn-Al-Si-Sn alloyfor hot-dip coating having a composition of 2-20 wt% Al, 0.001-0.5 wt%Si, 0.01-0.1 wt% Sn and remainder Zn is proposed. In both of thesecoatings, however, highly Al rich bata (β) phase precipitates as primarycrystals in the alloy coating and in regions where the Al contentexceeds 5 wt% the corrosion resistance deteriorates. In particular, inthe case of the latter proposal, in regions where the Al level fallsbelow 5 wt%, the coating shows remarkably favorable corrosion weightloss in a salt water spray test, but by the influence of added Sn,intercrystalline corrosion appears and further adhesion between thecoating and the steel product is inferior and the coating peels off by2T bending test and tape test and consequently, the workability is bad.

The Japanese Patent Publication (before examination) No. Tokkaisho58-177446 (1983) discloses alloy coated steel sheets having goodcorrosion resistance and paintability and the coating consists of 3-40wt% Al, 0.05-2.0 wt% Mg, 0.015-4.0 wt% Si (0.005-0.1 times of Alcontent), less than 0.02 wt% lead (Pb) and the remainder Zn. Even inthis method, in the range where Al content exceeds 5 wt%, corrosionresistance deteriorates because Al--rich and grown beta-phaseprecipitates as primary crystals. On the other hand when Al contentfalls below 5 wt%, corrosion weight loss becomes very small in a saltwater spray test, however, intercrystalline corrosion occurs since Mg inthe alloy coating exceed the solubility limit in solid solution and thisMg serves to cause intercrystalline corrosion, rather than prevent it tomeet the purpose for which it is normally added.

On the other hand, there are dry methods and wet method for productionof hot-dip alloy coated steel by using a molten bath of zinc alloy. Indry method, non-annealed steel products have their surfaces chemicallyreduced by hydrogen and ammonia, etc., at a high-temperature in afurnace, and are then dipped in a coating bath at controlledtemperature. In the wet method, annealed steel products are degreased byalkali, acid-pickled, rinsed with water sufficiently, and fluxed andthen they are dipped in a coating bath. For fluxing, the steel productsare ordinally dipped in water solution of flux composition and dried toform the film of flux composition on the steel surface and then thesteel product with thin film is dipped in coating bath. In the coatingbath, flux composition melts and removes from the surface of steelproducts to expose the surface to and contact with melted zinc alloy.The dry method is an excellent method if the coating process isconnected with steelmaking and rolling sectors, but its application isdifficult when coating is applied for steel pipes and tubes, sectionsteel and shaped or contoured steel parts, etc. The wet method can beused either with steel sheets, steel wires, steel pipe and tubes,section steel and shaped or contoured steel part. In the wet method,however, it is difficult to maintain the steel surface in a stablecondition as in the dry method, and as a result, alloy coatings havedefects such as un-coating. In order to prevent the defect ofun-coating, fluxing is performed before coating, but the fluxesgenerally used, such as ammonium chloride (NH₄ Cl) and ammonium zincchloride (ZnCl₂.3NH₄ Cl) can be used for hot-dip zinc alloys of anycompositions. However, when ZnCl₂.3NH₄ Cl is used for Zn-Al alloycoating, un-coating can not be avoidable.

SUMMARY OF THE INVENTION

It is an object of this invention to provide hot-dip zinc alloy coatedsteel products having an excellent corrosion resistance.

Another object of this invention is to provide hot-dip zinc alloy coatedsteel products having excellent resistance to intercrystallinecorrosion.

Another object of this invention is to provide hot-dip zinc alloy coatedsteel covered with coating having excellent adhesion property to steelproducts.

Another object of this invention is to provide flux compositionssuitable for hot-dip zinc alloy coating of steel products.

These objects, features and advantages of this invention will becomemore apparent in the detailed description and examples which follow.

FIGS. 1(a), (b) and (c) show vertical sectional views of electronmicroscopic observation at 2,200×, 2,000× and 1,700× of magnification,respectively, of the microstructures of alloy coatings.

FIG. 2 shows curves describing between the average corrosion weight lossper unit period of time (g/m² hr: ordinate) and Al content in coating(wt%: abscissa).

FIG. 3 shows the relation between corrosion weight loss (g/m² :ordinate) and the salt water spray test period (hr: abscissa).

FIG. 4 shows the relation between the dipping period (hr: abscissa) forwhich the steel product was dipped in the coating bath and the quantityof Fe dissolution (g: ordinate) in the bath.

FIG. 5 shows the relation between dipping period (hr: abscissa) of thesteel product in the coating bath and Al content (wt%: ordinate) of thebath.

FIG. 6 shows the relation between Fe content (wt%: abscissa) of thecoating bath and the corrosion weight loss (g/m² hr: ordinate) of thecoating by salt water spray test.

This invention is a novel hot-dip zinc alloy coated steel product coatedwith a Zn-Al-Si-Mg alloy having a composition of between 3.5 and 5.0 wt%Al, between 0.02 and 0.5 wt% Si, between 0.01 and less than 0.05 wt% Mg,and remainder Zn with unavoidable impurities between 0.01 and 0.04%.

In this invention, the alloy coating made on steel consists of Zn-richalpha (α) phase and alpha+beta eutectic phase, as shown in thephotograph of electron microscope in FIG. 1(a) attached hereto andsometimes has Si-rich 3rd phase but does not have Fe-Zn alloy layerswhich are ordinarily formed in hot-dip zinc alloy coatings.

Alpha phase consists of 0.05-1.0 wt% Al, 0.03-0.08 wt% Si, 0.01-0.05 wt%Mg and remainder Zn with unavoidable impurities and alpha+beta eutecticphase consists of 3.1-4.2 wt% Al, 0.02-0.07 wt% Si, 0.01-0.08 wt% Mg andremaining Zn with unavoidable impurities. The ratio of alpha phaseeutectic phase may increase to at most 37.5%, alpha+beta eutectic phasemay decrease to at least 62.5%. Si-rich 3rd phase having an area shareof 0-0.4% occasionally occurs, as far as, the average compositions ofthe coating are 3.5-5.0 wt% Al, 0.02-0.5 wt% Si, 0.01--less than 0.05wt% Mg and remaining Zn with a slight amount, namely 0-0.02 wt% at most,of unavoidable impurities. Both alpha phase and alpha+beta eutecticphase characteristically contain Mg in them in solid solution, below asolution limit of 0.05 wt% and 0.08 wt%, respectively.

Hot-dip zinc alloy coated steel products in this invention include steelsheets, steel pipes and tubes, steel wires, section steel and shaped orcontoured steel parts, such as bolts and nuts and all of them are coatedwith above described alloy. The coating in this invention can beperformed by dipping the steel products in a coating bath held at450°-480° C., preferably at 460°-480° C. and consisting essentially ofthe same average composition of which the aforesaid alloy coating isformed, after this, pulling them up from the bath and allowing them tostand to cool in the atmosphere or being rapidly cooled.

In fluxing, for the coating in this invention, a water solution of200-300 g/liter-water of fluxing compositions, the fluxing compositionconsisting of 0.5-8.0 wt% SnCl₂, 0.5-8.0 wt% acid fluorides, such as NH₄HF₂, NaHF₂ and KHF₂, 5-30 wt% alkali chlorides, such as NaCl and KCl,and remainder ZnCl₂ is preferably used to enable a superb alloy coating.

In this invention, the alloy coating has markedly improved corrosionresistance as expressed in terms of corrosion weight loss under a saltwater spray test and can completely inhibit intercrystalline corrosion.As shown in FIG. 1(a), the alloy coating has no Fe-Zn alloy layer atall, however, as shown in the tape test in Table 3, it has excellentadherence properties with steel.

When the Al content in the average compositions of the alloy coatingexceeds 5 wt%, Al-rich beta phase precipitates as primary crystals tocause a drop in corrosion resistance and adhesive property. As shown inFIG. 2, at a range of 3.5-5.0 wt% Al the corrosion resistance asexpressed by corrosion weight loss under a salt water spray test reachesthe lowest level and in light of this, a range of Al composition isselected at 3.5-5.0 wt%.

It is known that Si acts to improve the workability of hot-dip zincalloy coated steel products. However, in this invention, as shown inFIG. 2 Si also contributes to improve corrosion resistance. The upperlimit of Si content is decided at 0.5 wt% from a view point that Siprecipitates at 5 wt% Al as primary crystals to cause a drop incorrosion resistance and the lower limit of Si is 0.02 wt% from a viewpoint that less than 0.002 wt% Si can not improve corrosion resistance.In cases that Al content falls below 5 wt%, any amount of Si in excessof the solid solubility limit in solid solution precipitates as Si-rich3rd phase, but how this affects corrosion resistivity is yet to beclarified.

It is known that Mg is added to improve intercrystalline corrosion,however, in this invention, Mg contributes to improve corrosionresistance for salt water spray test as shown in FIG. 2. Mg shows aneffect to restrain intercrystalline corrosion to the extent that Mg incoatings completely dissolves in alpha phase and alpha+beta phase as asolid solution, however, when Mg exceeds solubility limit, Mg promotesintercrystalline corrosion as shown in FIG. 1(b). At 0.05 wt% or moreMg, especially 0.06 wt% Mg, Mg is concentrated around alpha phasegranules, and less than 0.05 wt% Mg restrains intercrystallinecorrosion. The upper off limit of Mg content, therefore, is 0.05 wt%,and the upper boundary becomes less than 0.05 wt%. For the sake ofallowance in solubility and the efficacy of Mg component, for example,0.04 wt% is the preferable concentration of Mg. The lower limit of Mgshould desirably be set at no less than 0.01 wt% at the least, thoughthis differs with the level of unavoidable impurities, such as lead,copper and tin, which cause intercrystalline corrosion in coatings.

In this invention, the wet method does not limit the shape of the steelmaterials to be coated and keeps plant cost and coating cost down.

The composition of the coating bath used for manufacturing coated steelproducts in the invention is almost free from changes of compositionaccording to time lapse and is therefore stable as shown in FIG. 5. Inother words, the same composition of alloy coating as that of thecoating bath can be stably obtained.

Furthermore, as shown in FIG. 4, Fe dissolution into the bath isvirtually absent, and as shown in FIG. 6 Fe content, as unavoidableimpurities, of the coating bath, exerts a large effect on the corrosionweight loss in a salt water spray test. For this reason, the content ofFe in a coating bath should desirably be held below 0.02 wt%. Asdescribed before, the absence of Fe dissolution into the coating bathand that of changes of composition of the coating bath make the controlof the coating bath exceedingly easy and also alloy coating withexcellent corrosion resistance can be stably obtained.

Depending on the coating bath composition, at a coating bath temperaturelower than 450° C., the fluidity of the bath becomes too small andformation of a smooth coating surface becomes difficult, and at higherthan 490° C. of the coating bath temperature, Fe dissolution into thebath becomes too large, and consequently, the coating bath temperatureshould be kept 450°-480° C., and desirably, 460°-480° C.

The conditions under which alloy coated steel products are cooled afterpulling out from the coating bath, exert influence on the microstructureof the coating. Heat treatment after pulling out from the coating bathdeteriorates corrosion resistance because alpha+beta eutectic phase, inparticular, beta phase in the coating, develops too large coarseparticles. Further, the distribution of Mg in solid solution changes andthe inhibitory effect of Mg on intercrystalline corrosion decreases.Worse still, brittle Fe-Zn alloy phase appears in the coating and theworkability of the coated steel products deteriorates. For thesereasons, cooling conditions of standing to cool in the atmosphere orrapid cooling are preferred for this invention.

In the wet method it is well known that the fluxing removes oxides fromthe steel surface, improves wetting property between the steel productand hot-dip alloy and activates the steel surface to form a highlyadhesive coating. Accordingly, fluxes are required to act as detergentand as activator in relation to the steel surfaces. Simultaneously, thefluxes are required to form a suitable thin film on the surfaces of thesteel surface prior to their hot-dipping, said thin film, upon beingdipped in the coating bath, are required to readily liberate itself fromthe steel surface to expose the cleaned and activated steel surfacesdirectly to the coating bath. Fluxes are also required not tocontaminate the coating bath when they dissolve in the hot bath. If theyare able to perform any and all of these actions, any fluxingcomposition may be employed and the new fluxing compositions describedherein is preferably used for the coating in this invention.

In the new fluxing composition, SnCl₂ is used as the major ingredient.SnCl₂ has an exceedingly strong surface activating action. It functionsto increase the affinity between steel and the molten zinc alloy andimproves adhesive strength of the coating. SnCl₂, however, easilyhydrolyzes in the water solution. Acid fluorides such as NH₄ HF₂, NaHF₂and KHF₂ are employed to prevent hydrolysis of SnCl₂ in the watersolution, and further they possess marvelous fluxing activity. ZnCl₂ isknown as an important fluxing element and in fact possesses fluxingactivity. In this invention, it is an indispensable element to lower themelting point of a fluxing composition and make films of fluxcomposition on steel surface easy to expose the surface in molten zincalloy in the bath. An alkali chloride, such as NaCl and KCl, does notexhibit fluxing actions, but when added in a molecular ratio close tothe one in which ZnCl₂ is mixed to form an eutectic mixture, then suchas alkali chloride further lowers the melting point of the flux andpromotes the fluxing activity of other fluxing elements.

For the flux compositions, a content of SnCl₂ less than 0.5 wt% providesonly insufficient surface activity and when it exceeds 8 wt%, theprevention of hydrolysis of SnCl₂ in the water solution is undesirablyrendered difficult. Therefore, the content of SnCl₂ should be 0.5-8.0wt%, and for economic considerations, should desirably be 1-5 wt%.

The minimum amount of an acid fluoride needed to prevent hydrolysis ofSnCl₂ is equivalent to the amount of the SnCl₂. Accordingly, thecompositional range of acid fluorides is 0.5-8.0 wt%. In excess of 8.0wt%, acid fluorides undesirably cause precipitate of zinc fluoride inthe water solution.

Alkali chloride, below 5 wt% insufficiently lowers the melting point ofthe flux, and in excess of 30 wt%, it raises the melting point of theflux. Thus, the alkali chloride must compositionally be 5-30 wt%, ormore desirably, 10-20 wt%. Though it depends on the composition of otheringredients, ZnCl₂ must equal or exceed 50 wt% to obtain desired effect.

The fluxing composition in this invention is used as a water solution of200-300 g/liter and the water solution is an acid aqueous solutionhaving a pH value of 4.0-4.5 and its corrosivity to iron is excessivelysmall at 0.003-0.004 g/m² hr. Accordingly, the dissolution of iron ionsinto said water solution is so small that it becomes possible to controlcontents of flux composition in a water solution by measuring thespecific gravity of the solution. To flux steel products, steel productsare degreased, pickled and water-washed as usual and then are dipped ina water solution of flux composition for 30-60 seconds and dried up.

Then the steel products adhering flux on the steel surface are dipped incoating bath and flux on the surface swiftly melts, floats and partlyevaporates at the bath surface.

Fluorine in the flux compositions added as acid fluoride reacts with Znand Al, etc., and remains on the bath surface in the form of ZnF₂, AlF₂or other stable compounds. Generation of hydrogen fluoride gas and othervapor that become sources of environmental pollution are negligibly ofsmall amount.

The fluxing compositions of this invention not only can be used for thehot-dip zinc alloy coated steel products of this invention also showtheir excellent performance in fluxing operation of wet method forconventional hot-dip zinc alloy coated steel products.

To further illustrate this invention following examples and tests aregiven, but this invention is not limitted by those.

EXAMPLES

Alloys, zinc base alloys, were separately fused in graphite cruciblesset in a small electric furnace, and thus coating baths were prepared.Table 1 shows the coating bath compositions used.

0.3 mm thick cold-rolled steel sheets were degreased by a water solutionof 6 wt% caustic soda and were rinsed with water. After this, the sheetswere pickled in 8 wt% hydrochloric acid solution and were then rinsedwith water. The steel sheets were next dipped for 60 seconds in a watersolution of the fluxing composition as shown in Table 2. After thisfluxing operation, the sheets were held at a temperature of 200° C. fordrying. As comparative examples of the flux, as shown as Symbol F-7 inTable 2, a conventionally used water solution of ZnCl₂.3NH₄ Cl wasemployed.

The fluxed steel sheets were then dipped for 10 seconds in a variety ofcoating baths mentioned above respectively. The products were thenpulled up from the bath as hot-dip zinc alloy coated steel sheets.

The section of the hot-dip zinc alloy coated sheet obtained was thenexamined by EPMA (made by Electron Probe Micro Analyzer of Hitachi,Ltd.: 650×) to analyze the alloy coating for microstructure and forcomposition. The hot-dip zinc alloy coated steel obtained was alsomeasured for the coating weight of alloy by using the antimony chloridemethod (JIS-(Japanese Industrial Standard)-HO401).

Table 1 shows fluxing conditions, coating bath temperatures, coolingconditions after coating, coating weight of alloy, and the result of theanalysis of the alloy coating in addition to coating bath compositions.

Test

The hot-dip zinc alloy coating steel products obtained were put to thefollowing tests.

(1) Salt water spray test

A salt water spray test, in which 5% of salt water was sprayed at 35° C.and 95% of relative humidity, was conducted in accordance withJIS(Japanese Industrial Standard)-Z 2371. Table 3 shows the averagecorrosion weight loss per unit period of time and 5% red rust generationtime of the coating subjected to this test.

FIG. 1 shows the state of intercrystalline corrosion of alloy coatingafter 900 hours of salt water spray test. FIG. 1(a) shows coating ofthis invention (E-1) and from the photegraph, it became obvious thatintercrystalline corrosion did not occur. FIG. 1(b) shows coating ofcomparative example (C-4) and FIG. 1(c) shows coating of comparativeexample (C-5) and those two show that intercrystalline corrosionconsiderably progressed.

FIG. 2 shows relation between the average corrosion weight loss per unitperiod of time and Al content in coating at 72 hours salt water spraytest. FIG. 2 shows that addition of Si and further addition of Mgimprove corrosion resistance.

FIG. 3 shows the relation between corrosion weight loss and the saltwater spray test period and shows that coating (E-1 and E-2) of thisinvention and comparative example (C-2) are excellent in weight loss.However comparative example (C-2) has inferior corrosion resistancebecause it rusts at 1,200 hour spray of salt water (see table 3).

(2) Steam test

The alloy coated steels were exposed for three days in a saturated steamat 98° C. and, soon after this, were subjected to 2T bending test andtheir appearance was observed. And then tape test were carried out.Table 3 shows the results.

2T bending test: Coated sheet is folded twice at a curvature of twotimes thickness of the sheet and appearance of cracks on coating ofbended part is observed.

Tape test: After 2T bending test, adhesive tape is put on the coating ofthe bended part, the tape is repidly peeled off and peeling of thecoating is observed.

(3) Fe dissolution test

The same fluxed steel products having 30 cm² of surface as used in theexamples and in the comparative examples were dipped for an extendedperiod of time in 1.4 liter of the coating bath. After this, the coatingbath was checked for Fe dissolution, and for a change in Al content. Thehot-dip zinc alloy coated steel products thus obtained after a longperiod of dipping were then carried out salt water spray test. Theresults were shown in FIGS. 4, 5 and 6.

    TABLE 1          Alloy  Fluxing  Bath Cooling coating Composi- Concentra- Coating     bath composition (wt %) tempera- con- weight Alloy coating composition     (wt %)  No. tion*1 tion (g/l) Al Si Mg Sn Pb Fe Zn ature (°C.)     ditions (g/m.sup.2) Al Si Mg Sn Pb Fe Zn Note       Examples of this E-1 F-3 300 4.5 0.25 0.01 -- -- -- Remain- 480     Cooling  96 4.5 0.23 0.01 -- -- -- Remain-  *In E-5 and E-6 invention           ing  at at-        ing  Si-rich 3rd             mosphere     phases are found  E-2 F-3 200 4.5 0.25 0.04 -- -- -- Remain- 480 Cooling      98 4.5 0.24 0.04 -- -- -- Remain-  in addition to           ing  at at-            ing  α phases and             mosphere          α +     β phases.  E-3 F-4 250 4.5 0.13 0.01 -- -- -- Remain- 480 Rapid 292     4.5 0.13 0.01 -- -- -- Remain-  *Solute Si is           ing  Cooling        ing  present within  E-4 F-5 200 4.5 0.13 0.04 -- -- -- Remain- 480     Rapid 272 4.5 0.13 0.04 -- -- -- Remain-  phases at a           ing     cooling        ing  content of  E-5 F-1 300 3.5 0.25 0.01 -- -- --     Remain- 460 Cooling 146 3.5 0.24 0.01 -- -- -- Remain-  about 0.07%.           ing  at at-        ing  Mg is almost             mosphere     evenly dis-  E-6 F-2 300 3.5 0.25 0.04 -- -- -- Remain- 460 Rapid 152     3.5 0.24 0.04 -- -- -- Remain-  tributed in α           ing     cooling        ing  phases and  E-7 F-6 200 4.5 0.25 0.04 -- -- 0.02     Remain- 480 Cooling 188 4.5 0.25 0.04 -- -- 0.015 Remain-  α +     β phases.           ing  at at-        ing             mosphere     E-8 F-3 250 3.5 0.13 0.01 -- -- 0.01 Remain- 460 Cooling 246 3.5 0.13     0.01 -- -- 0.01  Remain-           ing  at at-        ing     mosphere Comparatives C-1 F-3 300 4.5 -- -- -- -- -- Remain- 480 Cooling     208 4.5 -- -- -- -- 0.02  Remain-           ing  at at-        ing           mosphere  C-2 F-3 300 4.5 0.13 -- -- -- -- Remain- 480 Cooling 234     4.5 0.13 -- -- -- 0.005 Remain-           ing  at at-        ing         mosphere  C-3 F-3 300 4.5 -- 0.04 -- -- -- Remain- 480 Cooling 260     4.5 -- 0.04 -- -- 0.005 Remain-           ing  at at-        ing         mosphere  C-4 F-3 300 4.5 0.13 0.10 -- -- -- Remain- 480 Cooling 220     4.5 0.13 0.10 -- -- -- Remain-           ing  at at-        ing        mosphere  C-5 F-3 300 4.5 0.13 -- 0.1 -- -- Remain- 480 Cooling 261     4.5 0.13 -- 0.1 -- 0.01  Remain-  Sn is not find           ing  at at-          ing  within             mosphere          phases.  C-6 F-3 300 0.15     -- 0.5  -- 0.03 0.02 Remain- 520 Cooling  98  0.15 -- 0.5  -- 0.03 0.05     Remain-           ing  at at-        ing             mosphere  *2 F-7     300 4.5 0.25 0.04 -- -- -- Remain- 480 Cooling -- -- -- -- -- -- -- --     Coating fails  C-7         ing  at at-          to form and     mosphere          alloy coating                       can not be made.     *2 F-7 300 3.5 0.13 0.01 -- -- -- Remain- 460 Cooling -- -- --  -- -- --     -- --  C-8         ing  at at-             mosphere     *1 Compositions are denoted in symbols of fluxing composition shown in     Table 2     *2 Comparative examples of both fluxing compositions C7 and C8

                  TABLE 2                                                         ______________________________________                                                                      Alkali                                          Sym-         Acid fluorides   chlorides                                       bol  SnCl.sub.2                                                                            NH.sub.4 HF.sub.2                                                                      NaHF.sub.2                                                                          KHF.sub.2                                                                           NaCl  KCl.sub.2                                                                          ZnCl.sub.2                       ______________________________________                                        F-1  0.5     0.5      --    --    20    --   79                               F-2  1.0     1.0      --    --    10    --   88                               F-3  2.0     2.0      --    --    10    --   86                               F-4  2.0     2.0      --    --    --    30   66                               F-5  4.0     --       4.0   --    --    10   82                               F-6  6.0     --       --    6.0   10    --   78                               F-7  Goods on the market: ZnCl.sub.2.3NH.sub.4 Cl                                  unit: wt %                                                               ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________           Salt water spray test                                                                         5% red                                                        Corrosion weight loss                                                                         rust gen-  Steam test                                  Sample (g/cm.sup.2 hr) eration                                                                             Appear-                                                                            2T bend-                                                                            Tape                                  No.                                                                              No.*                                                                              72 hrs                                                                            240 hrs                                                                           480 hrs                                                                           900 hrs                                                                           time (hr)                                                                           ance ing test                                                                            test                                  __________________________________________________________________________    1  E-1 0.05                                                                              0.027                                                                             0.017                                                                             0.010                                                                             more than                                                                           Good***                                                                            No cracks                                                                           No peeling                                                   4000                                                   2  E-2 0.04                                                                              0.025                                                                             0.016                                                                             0.010                                                                             more than                                                                           Good No cracks                                                                           No peeling                                                   4000                                                   3  E-3 0.07                                                                               --**                                                                             --  --  more than                                                                           Good No cracks                                                                           No peeling                                                   4000                                                   4  E-4 0.06                                                                              --  --  --  more than                                                                           Good No cracks                                                                           No peeling                                                   4000                                                   5  E-5 0.08                                                                              --  --  --  more than                                                                           Good No cracks                                                                           No peeling                                                   4000                                                   6  E-6 0.07                                                                              --  --  --  more than                                                                           Good No cracks                                                                           No peeling                                                   4000                                                   7  E-7 0.05                                                                              --  --  --  more than                                                                           Good No cracks                                                                           No peeling                                                   4000                                                   8  E-8 0.09                                                                              --  --  --  more than                                                                           Good No cracks                                                                           No peeling                                                   4000                                                   9  C-1 0.24                                                                              --  --  --  less than                                                                           Fair****                                                                           Partial                                                                             All peeled                                                    500       cracking                                                                            off                                   10 C-2 0.06                                                                              0.029                                                                             0.019                                                                             0.011                                                                             less than                                                                           Fair No cracks                                                                           Partially                                                    1200             peeled                                11 C-3 0.12                                                                              0.040                                                                             0.028                                                                             --  less than                                                                           Fair Partical                                                                            All peeled                                                    900       cracking                                                                            off                                   12 C-4 0.06                                                                              0.030                                                                             0.021                                                                             --  less than                                                                           Good No cracks                                                                           Partially                                                     900             peeled                                13 C-5 0.09                                                                              0.066                                                                             --  --  less than                                                                           Good General                                                                             All peeled                                                    480       cracking                                                                            off                                   14 C-6 0.5 0.300                                                                             0.170                                                                             --  less than                                                                           Fair Partical                                                                            All peeled                                                    250       cracking                                                                            off                                   __________________________________________________________________________     *The sample numbers represent the numbers of working examples and of          comparative examples given in Table 1.                                        **The empty spaces in the table signify that measurements have not been       taken.                                                                        ***"Good" means that appearance of coating surface is almost unchanged.       ****"Fair" means that no spangle appears of the coating surface.         

We claim:
 1. A zinc alloy coated steel product coated with a Zn-Al-Si-Mgalloy having a composition consisting of between 3.5 and 5.0 wt%aluminum, between 0.02 and 0.5 wt% silicon, between 0.01 and less than0.05 wt% magnesium and remainder zinc and unavoidable impurities, saidproduct being capable of withstanding, after exposure to saturated steamfor three days at 98° C., a 2T bending test without cracking and a tapetest without peeling.
 2. The zinc alloy coated steel product of claim 1,wherein said composition contains between 0.01 and 0.04 wt% magnesium.3. The zinc alloy coated steel product of claim 1, wherein saidcomposition contains less than 0.02 wt% iron as an unavoidable impurity.